This invention relates to flow sensors for fluids, and more particularly to such sensors in which a heater is maintained at a constant temperature differential above the temperature of the flowing fluid.
Modern ships employ crew members whose function is to monitor various parts of the vessel, and to operate equipment such as hoists, radar, bridge equipment, and to monitor and control valves located throughout the ship. The costs associated with maintaining a large crew are disadvantageous, and such costs include the costs associated with paying wages, maintaining the crew member in terms of food and life support (bathrooms, hot water, and the like), and also includes the costs of training the crew member for the particular job. To the extent that a ship""s functions can be automated, the necessary crew can be reduced.
The problem is particularly acute in war vessels, as a relatively large crew must be maintained in order to have the resources to perform battle damage repair and recovery.
If reliable and inexpensive flow sensors were available, such sensors could be located in various pipes within a ship or a factory, and their readings could be compared to determine if there were a break in the intervening pipe or flow path. Such inexpensive sensors could also be used to improve process controls in chemical and other processes. Present-day flow sensors include rotating-propeller or linear types, differential-pressure aperture, ball-in-tapered-tube, vane or deflection type, ultrasonic, and hot-wire anemometer. The rotating-propeller is very accurate, but may degrade over time as a function of corrosion and deposits, and may fail catastrophically in the presence of large debris. The differential-pressure type of flow sensor requires an obstructing aperture or change of geometry of the flow path, which is very undesirable, and when the application requires many such sensors to be cascaded, may substantially impede the flow. Also, the small pressure changes attributable to relatively large apertures may undesirably introduce noise into the measurement. The ball-in-tube type requires a vertical orientation, and the tube must be transparent in order to optically detect the location of the ball. Additionally, in a vehicle which has vertical motion, the vertical acceleration tends to add to the gravitational force acting on the ball, and will tend to affect the reading, and therefore the accuracy. The vane deflection type of flow sensor obstructs the flow with the vane, and is not known for their accuracy. The ultrasonic type does not necessarily impede the flow, but is expensive, and may not be suitable for use in a noisy environment, or in an environment in which many such sensors are in use, so that the ultrasonic signals of one affect the others in the same flow path. The hot-wire anemometer is not known for use in fluids other than air, would not work in a conductive fluid, and the thin wire would be subject to breakage by circulating debris in some applications. Some approaches such as the propeller and differential pressure based ones impose stringent installation requirements such as a pipe having a length of many pipe diameters preceding the active portion, for flow straightening prior to the sensor; this may not be acceptable in some applications.
Improved flow sensors are desired.
A flow sensor according to an aspect of the invention includes a path for the flow of fluid in a region. The fluid should have a known specific heat or value of temperature change per unit mass per unit of energy. A temperature determining means or temperature sensor is coupled to the path for making a determination of the upstream temperature of a fluid flowing in the path, or at least the approximate upstream temperature should be known. Ordinarily, the temperature determination is made by a thermal sensor coupled to the flow path at an upstream location. A heating means or heater is coupled to the path, for heat transfer with the fluid. A control arrangement is coupled to the heater and to the temperature determining means, for applying power to the heater in an amount selected or required to raise the temperature of the heater above the upstream temperature by a predetermined amount, and for converting the value of the power into a corresponding flow value.
In a preferred embodiment of the invention, the control means includes a temperature determining means coupled to the heater, for determining the temperature of the heater. In a most preferred embodiment, the heater is an electrical resistor, and the heater temperature determining means comprises electrical resistance measuring means coupled to the heater for measuring the electrical resistance of the heater, and the control means comprises means for converting the value of the resistance into a corresponding temperature of the heater.
The control means preferably includes a memory preprogrammed with a value corresponding to the cross-sectional area of the path, and the flow determination is generated in the form of one of mass quantity per unit time and volume per unit time.
A method according to another aspect of the invention includes the steps of estimating, determining or setting the temperature of a fluid flowing in a path, and applying power to a heater thermally coupled to the flow for raising the temperature of the heater by a fixed temperature differential above the temperature of the fluid. Using at least information equivalent to or corresponding to the specific heat of the fluid, the exposed area of the heater, the amount of power required to sustain the temperature differential, the power transfer characteristics per unit area of the heater to the fluid, and the exposed area of the heater, determining the fluid flow. The flow may be given in terms of volumetric flow, by the use of the fluid flow together with information equivalent to the cross-sectional area of the path. The flow may be given in terms of mass flow, by the use of the volumetric flow, in combination with information equivalent to the mass density of the fluid.
By changing the memorized information relating to the physical characteristics of the fluid, the application of the flow sensor to different fluids can be changed.